It is well known that nylon 6.6 yarns have bi-elastic tensile properties, which means low initial modulus and higher final modulus. The low initial modulus makes the tire lifting/expansion process possible without any distortions in green tire, and high final modulus (modulus after transition point from low to high modulus) improves high speed tire durability as cap ply in pneumatic radial tires (PCR and LT).
In U.S. Pat. No. 3,343,363 single twisted nylon 6.6 strands/yarns and cords having initial modulus of between 25 and 60 g/d and tenacity higher than 7.0 g/d at room temperature have been disclosed as tire reinforcements. The initial modulus values are determined according to ASTM A1380-61T.
In U.S. Pat. No. 3,849,976 high modulus nylon 6.6 single-ply, two-ply and three-ply cords having L5 Modulus higher than 60 g/d obtained by applying high stretch at high temperature during hot stretching process have been disclosed as tire reinforcement.
In U.S. Pat. No. 4,284,117 single twisted nylon, polyester, rayon and aramid yarns are disclosed as cap ply reinforcements in pneumatic radial tires.
In order to obtain the high modulus single twisted yarns, the existing regular nylon 6.6 yarns having regular modulus (tensile stress level less than 1.25 cN/dtex at 4% elongation, or tensile modulus level at 4% elongation less than 31.25 cN/dtex or 35.4 g/d) have been used. Those single twisted yarns prepared from the regular modulus yarns had very high stiffness and lower bending fatigue resistance due to excessive stretching at high temperatures.